This invention relates generally to porous materials. In particular, this invention relates to the preparation and use of porous films containing thermoset materials and having a low dielectric constant.
As electronic devices become smaller, there is a continuing desire in the electronics industry to increase the circuit density in electronic components, e.g., integrated circuits, circuit boards, multichip modules, chip test devices, and the like without degrading electrical performance, e.g., crosstalk or capacitive coupling, and also to increase the speed of signal propagation in these components. One method of accomplishing these goals is to reduce the dielectric constant of the interlayer, or intermetal, insulating material used in the components. A method for reducing the dielectric constant of such interlayer, or intermetal, insulating material is to incorporate within the insulating film very small, uniformly dispersed pores or voids.
Porous dielectric matrix materials are well known in the art. One known process of making a porous dielectric involves co-polymerizing a thermally labile monomer with a dielectric monomer to form a block copolymer, followed by heating to decompose the thermally labile monomer unit. See, for example, U.S. Pat. No. 5,776,990. In this approach, the amount of the thermally labile monomer unit is limited to amounts less than about 30% by volume. If more than about 30% by volume of the thermally labile monomer is used, the resulting dielectric material has cylindrical or lamellar domains, instead of pores or voids, which lead to interconnected or collapsed structures upon removal, i.e., heating to degrade the thermally labile monomer unit. See, for example, Carter et. al., Polyimide Nanofoams from Phase-Separated Block Copolymers, Electrochemical Society Proceedings volume 97-8, pages 32-43 (1997). Thus, the block copolymer approach provides only a limited reduction in the dielectric constant of the matrix material.
Another known process for preparing porous dielectric materials disperses thermally removable particles in a dielectric precursor, polymerizing the dielectric precursor without substantially removing the particles, followed by heating to substantially remove the particles, and, if needed, completing the curing of the dielectric material. See, for example, U.S. Pat. No. 5,700,844. In the '844 patent, uniform pore sizes of 0.5 to 20 microns are achieved. However, this methodology is unsuitable for such electronic devices as integrated circuits where feature sizes are expected to go below 0.25 microns.
U.S. Pat. No. 6,420,441 (Allen et al.), discloses porogen particles that are substantially compatibilized with B-staged dielectric matrix materials. However, this patent application does not broadly teach how to prepare porous dielectric layers containing polyarylene materials.
Polyarylenes are well known dielectric materials. For example, U.S. Pat. No. 6,093,636 (Carter et al.) discloses a method for forming an integrated circuit containing a porous high temperature thermoset, such as a polyarylene. Such porous thermosets are prepared by using as pore forming material highly branched aliphatic esters that have functional groups that are further functionalized with appropriate reactive groups such that the functionalized aliphatic esters are incorporated into, i.e. copolymerized with, the vitrifying polymer matrix. Such incorporation of the pore forming material into the matrix restricts the mobility of the pore forming material, i.e. incorporation prevents phase separation of the pore forming material from the matrix. By restricting such mobility, the size of the phase-separated domains is also restricted. Also, the '636 patent does not teach how to prepare porous thermoset dielectric materials having a mean pore diameter ≦10 nm, such as a diameter in the range of 0.75 to 8 nm.
International Patent Application WO 00/31183 (Bruza et al.) discloses a porous cross-linked thermoset dielectric matrix material, such as a polyarylene. This patent application discloses a number of porogens, such as solvents and polymers, particularly cross-linkable polymers. Suitable cross-linkable polymers are those that react to copolymerize with the thermoset dielectric matrix material. Suitable polymers useful as porogens include dendrimers, hyperbranched polymer systems and cross-linked latex particles. This patent application does not teach how to prepare porous thermoset dielectric materials having a mean pore diameter ≦10 nm, such as a diameter in the range of 0.75 to 8 nm, nor how to prepare such porous materials where the porogens are substantially free of aggregation or agglomeration and without copolymerization with the dielectric matrix materials.
Other methods of preparing porous dielectric materials are known, but suffer from broad distributions of pore sizes, too large pore size, such as greater than 20 microns, or technologies that are too expensive for commercial use, such as liquid extractions under supercritical conditions.
There is thus a need for improved porous thermoset dielectric matrix materials with substantially smaller pore sizes and a greater percent by volume of pores for use in electronic components, and in particular, as an interlayer, or intermetal, dielectric material for use in the fabrication of integrated circuits. There is also a need for porous thermoset dielectric materials where the volume fraction of pores in the film is equivalent to the volume fraction of pore forming material.